Pyrodex or BP is a personal choice. You can buy Pyro in pellets or powder form. A charge of BP can be weighed on a grain scale but Pyro RS is only used with a measure. RS stands for Rifle /Shotgun. The P Pyro is for Pistol.
Pellets come in preformed charge weights. I'd suggest you start with RS Powder and a powder measure. Start about 60 Grains and work up in 5 grain steps until you hit a load that shoots fine in your gun. It's almost impossible to say what load will work best since all of them shoot different in different guns. Start about 15 yards so you should hit paper. This is to test your sights.When you are in the black move out to 25 and then 50 when you're hitting regularly. Use a solid rest so you don't shake while sighting in. When you have a good load for a 3-5 shot group stick with it. Pyro works best with hot caps. Hope this helps.
Rock Lock